Washing machines generally have drum-type washers that wash laundry with falling impact by dropping the laundry by rotation of the drum, pulsator-type washers that wash laundry with friction between the laundry and water by rotating a pulsator installed on the bottom of the drum, and agitator-type washers that wash laundry with frictional force between the laundry and water by using an agitator protruding from the inside of the drum to produce water currents.
Such various washing machines rotate e.g., the drum clockwise or counterclockwise at regular speed in order to spin water in washing and rinsing courses or agitate the laundry. On the other hand, in a spin-drying course, the washing machine rotates e.g., the drum at a high speed to apply centrifugal force to the laundry and separates water from the laundry using the centrifugal force.
Typically, the washing machine uses chemical actions by a detergent and mechanical actions by rotation of the drum, pulsator, agitator, etc., to separate and remove stains from the laundry.
Conventional washing machines rotate the drum at an optimal rotation speed designed beforehand to optimize the mechanical actions used in washing.
However, although the optimal rotation speed varies depending on types of the laundry, the pre-designed rotation speed has not reflected this.